Codel International Ltd

Getting the design right for a reliable CO mill fire monitoring system

13-07-2023

Potential coal fires and coal-induced explosions in a coal handling system pose serious safety concerns for any industry that is using pulverized coal.

Spontaneous combustion of coal in a coal handling system (coal conveyors, coal mills, coal silos, coal
bunkers) has long been recognized as a fire hazard. Coal fires usually start in hotspots when coal
absorbs oxygen from the air, heat generated by this process can then ignite a fire. Without
a complete combustion, smouldering of fuel will produce large amounts of CO gas.

The early detection of coal fires is essential to prevent catastrophic damage to expensive plant and serious injury to personnel. It only takes 2 kilos of Pulverised Fuel (PF) to start a fire. Continuous measurement of CO levels in critical areas is a well proven method in detecting potentially dangerous coal fires at early stage. In Silos the smouldering coal may be below the surface of the coal pile, the CO permeating up though the column gives an indication much earlier than any temperature reading, which is unable to read sub surface temperatures.

Sample Probe Filter

The Codel sample probe has a very low cost replaceable sintered filter disc. This disc sits slightly back from the gas flow and utilises the abusive nature of the PF to self-clean the filter surface. For this reason, this is an extremely long-life component. This is an important benefit because removing the sample probe from a running mill can only be done when the mill is offline, a longer lifetime results in fewer interruptions to the operation of the mill.

Condensate Removal

Peristaltic pumps are notorious for High maintenance and often suffer from split tubes. To improve this the condensate drain is automated using solenoids. This reduces the maintenance to zero and removes a potentially unreliable component.

The choice between Electro Chemical (EC) and Infrared (IR)

Both of these measurement techniques are available, and both have benefits.

Electro Chemical Cell

The EC cell has a fundamentally different operating state compared to the IR. The EC cell is a modified micro fuel cell (essentially a battery). The gas diffuses over the sensor and a current is generated. This device has a limited lifespan as the material inside are consumed by the measurement. The lifespan is dependant of the actual levels of gas being measured (higher readings equal shorter life). The target lifespan for a CODEL EC cell measuring 0-50 ppm is a realistic two years. As the sensor is consumed the output is reduced. For this reason, the EC cell needs to be recalibrated regularly. This will be more often if the client is measuring a very low (or zero) concentration of CO. The output voltage is zero at zero ppm CO, a dead cell is only detectable by the injection of CO gas. The lifetime of the cell can be maximised by controlling the temperature and humidity of the cell within its housing.

Infrared Absorption

The IR measurement assembly is a cross correlation device and is a heavy industrial unit. It is not a consumable and can be considered a lifetime component. It has a maximum transmission at zero ppm, hence the concern over dead cells does not apply here and the unit does not need calibrating quite so often. The IR measurement assembly has a much larger measuring range compared to any EC cell. Unlike IR an EC monitor will saturate quickly in the event of a fire and may not return to service for some time afterwards (over-range). Clients often wish to see the value which the CO reaches during the event, not just the fact that it has occurred.

Read more about CO coal mill applications here.

For more information on Getting the design right for a reliable CO mill fire monitoring system talk to Codel International Ltd